The present invention relates to a real-time, imaging contact gauge for indicating contact pressure distribution over a plane or curved surface, and also to a visual feedback interactive apparatus including such contact gauges.
Contact pressure distribution measurements are generally made by electronic or mechanical pressure transducers. However, such transducers are typically large and bulky; and even when they are miniaturized, it is still difficult to adapt them for use in a limited space because of their electrical and mechanical connections. In addition, the mere introduction of the transducers produces local stiffness changes in the contact surface which may affect the measurements. Further, using the known electronic and mechanical pressure transducers for indicating contact pressure distribution over a large surface generally involves a scanning process, and therefore does not permit simultaneous measurement of the pressure distribution over the complete surface. Still further, such known systems generally are very costly to produce and require a high level of technical personnel to operate and maintain them.
U.S. Pat. No. 3,966,326, of which one of the joint inventors of the present invention was also a joint inventor, disclosed a method and apparatus for indicating or measuring contact stress distribution over a surface by the use of a photoelastic (i.e., a birefringent) material having optical properties which are changed when the member is subjected to mechanical pressure. Photoelastic (birefringent) sheets had previously been used for indicating or measuring stresses on the outer areas of parts and structures manifested by the changes in the index of refraction of such materials when subjected to stresses. Thus, a polarized light wave passing through the stressed material splits into two separate light waves, each vibrating along a principal stress direction, and each travelling at a different speed. A "phase shift" between the two light waves is produced by the stresses, resulting in the production of an optical interference pattern which can be displayed and recorded. When such a stress sensitive material is subjected to in-plane strains and is viewed under polarized light, the resulting stresses are seen as colored interference patterns which can be interpreted to indicate the overall stress distribution and to provide accurate measurements of the stress directions and magnitudes.
Previous to the above-cited U.S. Pat. No. 3,966,326, photoelastic sheets had been used only for measuring in-plane stresses, i.e., stresses produced in the material by forces substantially coplanar to the sheet itself. The invention described in the above patent provided means, namely a pressure-transmitting member including a plurality of point-projections, which were effective to transmit the contact pressure in the form of a plurality of localized points to the photoelastic (birefringent) member, thereby converting the pressures perpendicular to the material into "in-plane" stresses coplanar to the material.